Adhesives are made up of various components such as polymers, tackifiers, waxes and oils. Adhesive formulations based on these ingredients are susceptible to degradation. The consequences of degradation are discoloration, loss of elongation, loss of tensile strength, loss of tack and change in viscosity, molecular weight and molecular weight distribution. Degradation can be caused by prolonged exposure to sunlight. Sunlight contains invisible ultraviolet (UV) radiation with wavelengths between 290 and 400 nm. This radiation is responsible for the initiation of photodegradation.
Absorption of UV light by chromophores present in the adhesive formulation transforms the chromophores into their excited states which can undergo further undesired reactions. Some polymers contain strongly absorbing chromophores as a major part of their structures. Other polymers contain unintentional impurities such as ketones and hydroperoxide moieties and catalyst residues which act as chromophores. Absorption of UV radiation by these chromophores eventually results in bond cleavage, chain scission and/or crosslinking reactions.
Photostabilization of adhesives can be achieved by the addition of UV absorbers which convert the absorbed energy into harmless heat. An ideal UV absorber should be extremely photostable and have increased absorption over the UV range from 290 to 400 nm, but particularly the range of 350 to 400 nm. Classes of UV absorbers include the salicylates, cyanoacrylates, malonates, oxanilides, benzophenones, s-triazines and benzotriazoles.
Salicylates, cyanoacrylates, malonates and oxanilides absorb UV light primarily at the lower wavelengths of the UV range. These compounds have little to no absorption in the range of 350 to 400 nm which make them unsuitable for the instant applications. Benzophenones absorb over the lower half of the UV range, and they tend to be prone to yellowing upon light exposure due to photodegradation. Recently, it has been shown photochemically that benzophenones decompose prematurely in ethylene-vinyl acetate encapsulants which lead to the production of polyenic chromophores. This color generation from light yellow to brown is not only highly undesirable and unsightly in adhesive systems, but also can results in a loss of adhesive properties. By contrast, selected s-triazine UV absorbers are particularly useful because of their increased photostability.
Some polymers such as polycarbonates, polyesters and aromatic polyurethanes contain strongly absorbing chromophores as a major and integral part of their structures. Poly(ethylene terephthalate) (PET) and poly(ethylene 2,6-naphthalenedicarboxylate) (PEN) are particular examples the latter of which absorbs into the red UV region and especially need red-shifted s-triazines for UV protection. Adding an adhesive UV screening layer containing the s-triazines especially those described in the instant invention, further protects such polymers in multilayered constructions and articles.
The description, preparation and uses of the s-triazine UV absorbers are described for automotive coatings, photographic application, polymeric film coatings and ink jet printing. Automotive coatings are described in British 2,317,174A and 2,317,893A and in U.S. Pat. Nos. 5,556,973; 5,681,955; 5,726,309 and 5,106,891. Photographic applications are disclosed in U.S. Pat. No. 3,843,371 and copending application Ser. No. 08/974,263. Polymeric film coatings are described in U.S. Pat. Nos. 4,619,956 and 4,740,542. Ink jet printing is disclosed in U.S. Pat. No. 5,096,489. From each of these patents, the s-triazines UV absorbers are revealed as very photostable.
The s-triazine UV absorbers can be prepared by the general synthetic procedures outlined in U.S. Pat. Nos. 5,726,309; 5,681,955 and 5,556,973; British 2,317,714A and WO 96/28431.
The use of UV absorbers in adhesives is known as well. U.S. Pat. Nos. 5,683,804; 5,387,458; 5,618,626 and 5,643,676 demonstrate the use of UV absorbers in the adhesive layers of various articles. Typically benzophenones, cyanoacrylates, benzotriazoles and salicylates are used. The benzophenones, cyanoacrylates and salicylates do not provide acceptable performance in these applications. The benzotriazoles lack the high molar extinction values exhibited by the instant s-triazine UV absorbers. Quite surprisingly, the instant s-triazines are amazingly soluble in the adhesives making them especially well-suited for use in these applications. The use of s-triazine UV absorbers in adhesive compositions is not known in the prior art.
It is known in the art that the concomitant use of a hindered amine light stabilizer with a UV absorber such as an s-triazine provides excellent stabilization in many polymer compositions as summarized by G. Berner and M. Rembold, "New Light Stabilizers for High Solids Coatings", Organic Coatings and Science and Technology, Vol. 6, Dekkar, N.Y., pp 55-85.
Molecules containing both a UV absorber moiety and a hindered amine moiety with N--H, N-alkyl, N-alkanoyl and N-hydrocarbyloxy derivatives are described in U.S. Pat. Nos. 4,289,686; 4,344,876; 4,426,471; 4,314,933; 4,481,315; 4,619,956 and 5,021,478; British 2,188,631; and L. Awar et al., "New Anti-UV Stabilizers for Automotive Coatings: (presented at the 1988 Annual Meeting of the Federation oif Societies for Coatings Technology).
The concomitant use of a separate hindered amine molecule and a separate UV absorber is also taught in U.S. Pat. No. 4,619,956.
U.S. Pat. No. 5,204,390 teaches pressure-sensitive, hot melt adhesives for application to plasticized surfaces. It is generically taught that benzotriazoles and 1,3,5-tris(2-hydroxyphenyl)-s-triazine might be used in such products.
The effect of UV light on laminated articles that are exposed to the sun or other sources of UV light are of great concern to the manufacturers of such articles. Over time, constant or repeated exposure to UV light can result in dye and/or pigment fade for dyes and/or pigments used in such articles and in the degradation or breakdown of the adhesives, polymers or other materials used in the construction of the articles. The aforementioned fading and degradation shorten the useful life of the articles in question, making protection from UV light exposure an issue of great importance to the manufacturers of such articles.
Molecules known as UV absorbers are generally known in the art. However, due to the differences discussed above between the various UV absorber classes, it is the s-triazines and articles containing them which will be discussed here. Due to the incompatibility and low solubility of certain UV absorbers, a need exists for a s-triazine UV absorber that is highly soluble and which is very photostable and with a high molar extinction value.
T. Nagashima and H. Kuramashi, J. Non-Cryst. Solids, 178, (1994), 182 report "Recently ultraviolet light (UV) shielding glass, which is UV absorbing over the range of long wavelengths (320-400 nm) to avoid sunburn effects, has become an important issue because of the possible hazard of skin cancer due to depletion of the ozone layer."
The use of s-triazine UV absorbers in adhesive compositions and articles addresses these concerns and is not known in the prior art. In addition, articles which incorporate the s-triazine UV absorber of the instant invention are useful in protecting interior textiles and fabrics from UV induced photodegradation.
U.S. Pat. No. 5,770,114 discloses stabilized compositions containing soluble benzotriazoles that are used in electrochromic devices. This patent discloses that 2-(2-hydroxy-5-methylphenyl)-2H-benzotriazole is effective in stabilizing such electrochromic devices against degradation from UV radiation, but that it is not soluble enough in aprotic solvents, e.g. propylene carbonate, to provide suitable protection for electrochromic devices for very long periods of time. This indicates that any acceptable UV absorber must also exhibit excellent solubility in substrates and solvents in order to be acceptable.
Articles which would benefit from the incorporation of the instant, highly soluble, high extinction and photostable s-triazine include, but are not limited to:
(a) Retroreflective Sheets and Signs and Conformable Marking Sheets as seen in WO 97/42261; and U.S. Pat. No. 5,387,458 which is incorporated herein by reference;
(b) Solar Control Films of Various Construction as seen in British 2,012,668; European 355,962; and U.S. Pat. Nos. 3,290,203; 3,681,179; 3,776,805 and 4,095,013 which are incorporated herein by reference;
(c) Corrosion Resistant Silver Mirrors and Solar Reflectors as seen in U.S. Pat. No. 4,645,714 which is incorporated herein by reference;
(d) Reflective Print Labels as seen in U.S. Pat. No. 5,564,843 which is incorporated herein by reference;
(e) UV Absorbing Glasses and Glass Coatings as seen in U.S. Pat. Nos. 5,372,889; 5,426,204; 5,683,804 and 5,618,626 which are incorporated herein by reference;
(f) Electrochromic Devices as seen in European 752,612 A1; and U.S. Pat. Nos. 5,239,406; 5,523,877 and 5,770,114 which are incorporated herein by reference;
(g) Films/Glazings as seen in WO 92/01557; Japanese Nos. 75-33286; 93-143668; 95-3217 and 96-143831; and U.S. Pat. No. 5,643,676 which is incorporated herein by reference;
(h) Windscreens and Intermediate Layers as seen in Japanese Nos. 80-40018; 90-192118; 90-335037; 90-335038; 92-110128 and 94-127591; and U.S. Pat. No. 5,618,863 which is incorporated herein by reference; and
(i) Optical Films as seen in WO 97/32225; and U.S. Pat. Nos. 4,871,784 and 5,217,794 which are incorporated herein by reference.